1. Field
The presently disclosed subject matter relates to a multi-color image forming apparatus such as a color electrophotographic printer, a color laser beam printer and a color print machine, and a method for detecting positioning color image patterns in a multi-color image forming apparatus.
2. Description of the Related Art
A first prior art multi-color image forming apparatus is constructed by a plurality of color image forming units for forming a plurality of different color images, a transfer belt to which positioning color image patterns are transferred by the color image forming units, an optical sensor for detecting the positioning color image patterns, and a control circuit for compensating for transfer alignment or registration of the color image forming units in accordance with the detected positioning color image patterns. The registration is required among the different color images to avoid a shift of color and change of hue. The positioning color image patterns are of a single structure (see: JP1-167769A). This will be explained later in detail.
In the above-described first prior art multi-color image forming apparatus, however, if the mounting angle of the optical sensor fluctuates, the detection accuracy of the positioning color image patterns would be reduced. As a result, the registration would be not completely compensated for.
A second prior art multi-color image forming apparatus includes positioning color image patterns of a double structure instead of those of a single structure (see: JP2007-114555A). This also will be explained later in detail.
Even in the above-described second prior art multi-color image forming apparatus, however, the detection accuracy of the positioning color image patterns is still low. As a result, the registration would be not completely compensated for either.